This invention relates to a fastening device made of a synthetic resin and comprising a female member provided with a flange, a leg extending outwardly from the flange and having an axially extended hole which is adapted to radially expand upon the insertion of a shank of a male member therein. The hole axially extends along the axis of the leg and the flange. The fastening device also comprises a male member provided with a head portion and a shaft portion extended outwardly from the head portion and adapted to be inserted into the hole or bore of the female member. The fastening device of the present invention is intended to fasten two panels together by passing the leg of the female member through apertures in the two panels, concentrically aligned when the panels are pressed tightly against each other, and then by forcibly driving the shaft portion of the male member into the bore of the female member thereby radially expanding a portion of and allowing the two panels to be squeezed between the expanded portion of the leg and the flange. A specific characteristic of the present invention is the fact that the circumferential surface of the shaft portion of the male member is externally threaded and at least one helical ridge, matched so as to threadably engage with the male member, is formed on the peripheral surface of the bore of the female member, whereby the insertion of the shaft portion of the male member into the bore is accomplished by axially thrusting the shaft portion, and the extraction of the shaft portion from the bore so as to separate the panels is accomplished by rotating the head portion of the male member thereby allowing the male member to be threadably disengaged with the female member.
It has been widely known that fasteners made of synthetic resin are extensively used in fitting together parts in automobiles and in assembling electrical appliances, etc. Among these known fasteners are included fasteners of the kind composed of two members, male and female, and designed to fasten two panels together. The most popular fastener of this kind is known to comprise a female member of synthetic resin provided with a flange, a leg extended outwardly from the inner side of the flange and split lengthwise so as to allow lateral or radial expansion, a hole extending along the axial length of the leg from the outer side of the flange to the longitudinal center of the leg, and a male member made of metal adapted to advance into the hole of the female member and, at the same time, radially expanding the leg. This fastener has a construction such that two panels are fastened together by inserting the leg of the female member through the aperture of one of the panels until the flange comes into contact with the inner side of the panel and then by mounting the other panel on the outer side of the flange and subsequently threading the male member into the hole or bore thereby fastening the latter panel to the outer side of the flange and, at the same time, causing the leg of the female member to expand radially within the aperture of the former panel. This fastener, therefore, entails a work of threading the male member as described above so as to fasten the two panels together.
This threading operation does not necessarily prove easy where numerous kinds of parts are fitted on products during an assembly line process. In an effort to overcome the difficulty, there has been proposed a fastener which uses a round bar-shaped male member of synthetic resin in place of the aforementioned externally threaded male member and which provides the fastening together of two panels by simply driving the male member forcibly into the hole or bore of the female member.
This forced entry type fastener, for the purpose of fastening parts, has advantages over the screw type fastener since it effects the fastening by simply driving the male member forcibly into the bore of the female member. On the other hand, it has a disadvantage that when parts fastened therewith are required to be replaced with new ones, the male member driven forcibly into the hole must be extracted therefrom which involves arduous work. As a solution to this difficulty, there has been proposed a fastener wherein the male member is provided with a portion adapted to project from the outer surface of the flange of the female member when the male member has been driven home fixedly positioned within the bore. When the two members of the fastener are to be separated from each other, the extraction of the male member is accomplished by conveniently axially pulling the projecting portion outward. Depending on the place at which the fastener is used, however, the protrusions of the projecting portion may prove inconvenient. Particularly where parts are fitted in automobiles and electrical appliances, the projecting portion may interfere with the fitting of other parts or impair the appearance of finished products. Also, the use of such fasteners may be impractical for reasons of poor safety.